A redox flow battery (hereinafter, may be referred to as “RF battery”) is one of the known large-capacity storage batteries (e.g., see PTLs 1 and 2). RF batteries commonly include a cell stack constituted by a plurality of sets of a cell frame, a positive electrode, a membrane, and a negative electrode which are stacked on top of one another. The cell frame includes a bipolar plate interposed between the positive electrode and the negative electrode and a frame body disposed on the periphery of the bipolar plate. In the cell stack, two bipolar plates of a pair of adjacent cell frames and positive and negative electrodes interposed between the bipolar electrodes with a membrane being interposed between the electrodes form a cell. An RF battery performs charge and discharge by circulating electrolytes through the cells that include the electrodes.
The electrodes of a redox flow battery serve as a reaction field that promotes the cell reaction of an active material (metal ions) included in the electrolytes. Carbon fiber aggregates that include carbon fibers (e.g., carbon felt) have been commonly used for producing the electrodes for redox flow batteries. The carbon fibers are typically PAN-based carbon fibers produced using polyacrylonitrile (PAN) fibers as a raw material (e.g., see PTLs 2 to 4).